This invention relates to an impact-resistant laminate structure, more particularly, it relates to a low density laminate having a toughened reinforced resin facing adhered to a syntactic foam core and the method for making the core and laminate.
For purposes herein, syntactic foams shall be defined as materials comprised of rigid, hollow microballoons adhered together with a bonding agent. Syntactic foams differ from blown foams, such as polystyrene foam or polyurethane foam, in that the cells in syntactic foams are formed by incorporation of small diameter, rigid microspheres into a bonding agent (typically, a resin binder) rather than by expansion with a blowing agent. It is also known in the art that short reinforcement fibers can be incorporated into the bonding agents, along with microspheres, in order to enhance structural and/or other desired properties.
The bonding agents generally used in the art are either thermosetting resins, or thermoplastic resin matrices. Some examples of the thermosetting resins used as bonding agents may be epoxy resins, bismaleimides, cyanates, unsaturated polyesters, non-cellular polyurethanes, thermosetting polyimides, and the like. Pertinent examples of the thermoplastic resin matrices used include polyaryletherketones, polyphenylenesulfide, polyimides, polyetherimides, and aromatic and aliphatic nylons, just to name a few. The microballoons (or microspheres) are generally rigid and hollow spheres of glass, carbon, polystyrene, or phenolic resins. The most commonly used or most commonly known hollow spheres in the syntactic foam art are glass microspheres (e.g., Scotchlite.TM. Glass Bubbles from 3M Company) offered commercially in particle densities ranging from .about.0.1 to 0.6 g/cc and diameters on the order of 5 to 200 microns.
Syntactic foams are prepared, basically, by mixing or dispersing microspheres with the thermosetting or thermoplastic bonding resin, and subsequent thermoforming ("curing" for thermosets, "press-molding" for thermoplastics) into a desired shape. However, since the microballoons are extremely small in size, and extremely lightweight in comparison to the densities of resins, the mixing stage is known to be fraught with difficulties which result in both process and product limitations. The various preparation methods disclosed in prior art and the specific process and product related limitations and problems are discussed below.
Preparation methods known in the art, in brief, are the following: 1) Dry blending of powder of the bonding agent (a thermoplastic resin or a thermoset, if the latter is solid) and the microspheres, followed by thermal press-bonding/shaping and/or curing. An alternative approach to this method, when using thermoplastics, is to add microspheres to a melt of the thermoplastic in an extruder or a kneader. 2) Mixing (or spray coating) of the microballoons in a liquid, where the liquid is either a low viscosity thermosetting resin or a solution of a thermoplastic matrix in an aprotic solvent, with a subsequent dry-off or evaporation of the solvent from the resulting pasty mass during the curing or molding process. 3) Preparation of thermoplastic syntactic foams by forming a slurry or a paste of the thermoplastic powder and microspheres in a volatile liquid which is a nonsolvent for the thermoplastic, as disclosed in U.S. Pat. No. 5,120,769.
There are process/product limitations and problems faced by the methods described above. More particularly, in the dry (solids) blending method, since the microspheres are extremely lightweight and very fine in size, a number of serious problems are encountered, namely: a) the microspheres take on the characteristic of a fine airborne dust, thus posing environmental problem; b) the dry mechanical/frictional mixing and rough handling causes significant breakage of the relatively fragile hollow glass microspheres; c) the higher density thermoplastic particles are prone to settling to the bottom of the container, thus causing non-uniformity in composition; and, d) it is practically impossible to make lower density (e.g., &lt;35 lbs/cu.ft.) syntactic foams which, in essence, are the most desired for weight/fuel cost savings in aerospace structural applications. This is due to the inability to mix-in higher amounts, e.g., any amounts higher than approximately 15-20% weight content of microspheres in the resin matrix, because of the very high bulk volume-to-weight ratio of the microballoons compared to the resin.
Prior art methods of mixing the microballoons (and other relevant additives, e.g., fibers, etc.) in a solution of the resin, as disclosed in U.S. Pat. No. 4,077,922, have the following drawbacks: Potential environmental safety and health hazards are posed by volatile solvents, used as a general rule for ease of evaporating them off during thermoforming. Defects and structural non-uniformities are created when the solvent is being driven off by heat from the interior of the syntactic foam, since the fast evaporating solvent tends to draw-off the resin from the surface of the microspheres at varying rates depending upon spatial location. This effect also makes it difficult to obtain batch-to-batch reproducibility. Another drawback of this method is the additional energy or cost associated with transporting the solvent-laden pasty mass to the molding or curing equipment. Solvent release and evaporation during this procedure is also possible which would pose environmental and health concerns for a commercial operation.
Syntactic foams preparation process route by way of forming a slurry or a paste of thermoplastic powder and microballoons in a volatile liquid which is a non-solvent for the thermoplastic, although minimizes some of the problems encountered in both the dry blending method and the solvent based mixing approach, it still maintains some of the same key drawbacks: (a) high cost of transportation, and/or health risk from prolonged-exposure to heavy solvent-laden pasty mass or slurry, (b) extra costs and environmental risks associated with use of large quantities of process additives needed such as thickeners, binders, surfactants and the like, and, (c) the limitation to the use of thermoplastic resins only in a form of fine ground powder form are some of the most obvious drawbacks.